In this celebrity-obsessed age of 24-hour social media and omnipresent paparazzi, it’s inconceivable that younger members of the royal family could mingle with us, the unwashed hoi polloi and chattering classes, without attracting attention.

Heirs to the throne would be engulfed by a sea of flashing smart phones, their every word regurgitated and scrutinised in 140 poorly punctuated characters.

Seventy years ago, Princess Elizabeth, the future Queen of England, and her sister Princess Margaret briefly escaped from Buckingham Palace to celebrate VE Day with the teeming crowds outside the royal residence. They mingled with their subjects, completely incognito, as the people of London marked the end of the Second World War with an exuberant evening of revelry. Screenwriters Trevor De Silva and Kevin Hood use this true event as the starting point for a heart-warming comedy of manners, which propels the two princesses on journeys of self-discovery in a capital awash with carnal desire and danger.

A Royal Night Out is frothy fun, embellishing fact with outlandish fiction under the jaunty direction of Julian Jarrold, who previously unbuttoned the stifled emotions of the era in the 2008 remake of Brideshead Revisited.

The film opens with archive footage of Winston Churchill announcing the end of the conflict with Germany. Jubilant crowds gather outside Buckingham Palace where King George VI (Rupert Everett) is preparing a radio address with encouragement from Queen Elizabeth (Emily Watson).

Their daughters, Princess Elizabeth (Sarah Gadon) and Princess Margaret (Bel Powley), yearn to celebrate with the people but the Queen is resistant.

‘We’ll be walled up in this mausoleum for the rest of our lives,’ despairs Margaret. ‘I’m completely cheesed!’

But there’s nothing here that will have the filmmakers entering the Tower Of London through Traitor’s Gate.